1. Field of the Invention
The present invention relates to a field emission device, and more particularly to a field emission device utilizing carbon nanotubes as emitters.
2. Description of Prior Art
Carbon nanotubes are a novel carbonaceous material discovered by Iijima, a researcher of NEC corporation, in 1991. Relevant information was reported in an article by Iijima, entitled “Helical Microtubules of Graphitic Carbon” (Nature, Vol.354, P56, 1991). Carbon nanotubes can transmit an extremely high electrical current and can emit electrons at a very low voltage of less than 100 volts, which makes it a very promising material for field emission applications. Walt A. de Heer et al. reported a field emission device employing carbon nanotubes as an electron emission source in an article entitled “A Carbon Nanotube Field-Emission Electron Source” (Science, Vol. 270, P1179-1180, 1995). Shoushan Fan et al. reported a method using a CVD (chemical vapor deposition) process for producing aligned parallel bundles of carbon nanotubes on a silicon or glass substrate in an article entitled “Self-oriented regular arrays of carbon nanotubes and their field emission properties” (Science, Vol. 283, P512-514, 1999).
Referring to FIG. 8, P. R. China patent application No. 00121140 discloses a field emission device using carbon nanotubes 90 as emitters. The carbon nanotubes 90 are mounted vertically on conductive polymer film (not labeled), fixed within corresponding fine holes 80 by simply putting the carbon nanotubes 90 into the fine holes 80. However, the carbon nanotubes 90 differ in height so that a top surface of the carbon nanotubes 90 is not flat. Therefore, the electron emissions from the emitters may be not uniform.
Referring to FIG. 9, U.S. Pat. No. 6,232,706, invented by Hongjie Dai et al., discloses a field emission device using aligned parallel bundles of carbon nanotubes 100 extending from patterned catalyst layers 130 deposited on a layer of porous silicon 120, which formed on a substrate 110. The bundles of carbon nanotubes 100 are formed using a chemical vapor deposition (CVD) process. However, the carbon nanotubes formed have a variety of heights, within a certain range, so that the bundles of carbon nanotubes can form a flat top, a bow-shaped top, or other shapes, which are neither predictable nor controllable. Furthermore, a thin layer, including disorderly nanotubes, a certain amount of remaining catalyst particles, and amorphous carbon material, may be produced on the tops of the bundles. All of the above defects may reduce the uniformity and stability of electron emissions from the bundles of carbon nanotubes.